In a low IF (Intermediate Frequency) type radio signal receiver system, which has been hitherto known, demodulated signals include, in principle, not only a desired actual signal but also an image signal as an unwanted component which is a frequency component having an opposite sign to that of the frequency component of the actual signal due to its generation mechanism. In order to remove the unwanted component as much as possible, a complex filter is used.
The complex filter has a function of allowing the actual signal to pass and attenuating the image signal. A gain in the frequency band of the actual signal of the complex filter is desirably higher than a gain in the frequency band of the image signal. A ratio between the gains is called an image rejection ratio. Accordingly, the complex filter is required to have a high image rejection ratio.
FIG. 10 shows an example of the radio signal receiver system as described above. In this receiver system, an LNA (Low Noise Amplifier) 111 amplifies a high-frequency received signal which is received by an antenna and quadrature-modulated. A quadrature mixer 1131 integrates the high-frequency received signal amplified by the LNA 111 and an output signal LO-I of a local oscillator (LO) 112. Further, a quadrature mixer 113Q integrates the high-frequency received signal amplified by the LNA 111 and an output signal LO-Q of the local oscillator 112. The output signal LO-Q is shifted in phase by 90 degrees from the output signal LO-I.
Thus, the high-frequency received signal is subjected to quadrature modulation while being down-converted to an intermediate frequency (IF band), thereby generating an IF-I signal and an IF-Q signal which are shifted in phase by 90 degrees from each other. A complex filter 114 removes image signals from the received IF-I signal and IF-Q signal.
A VGA (Variable Gain Amplifier) 115 amplifies the received signal to an appropriate amplitude. An ADC (Analog-Digital Converter) 116 converts the received analog signal into a digital signal. A digital signal processing unit 117 receives the converted digital signal and carries out various digital processing.
Though the image signals can be removed by using the complex filter as described above, it is necessary that the received IF-I signal and IF-Q signal have the same amplitude and have a phase of 90 degrees in order to secure a sufficient image rejection ratio.
However, due to a mismatch between elements or lines in the quadrature mixers or the complex filter, it is practically impossible to maintain the ideal relationship between the IF-I signal and the IF-Q signal. As shown in FIG. 11, this effect is considerably large in the image frequency band, and only a small mismatch considerably deteriorates the image rejection ratio.
A first related art for improving the image rejection ratio is disclosed in Patent Literature 1. In the first related art disclosed in Patent Literature 1, as shown in FIG. 12, IF band simulated image signals are input to a complex filter circuit 125, and an amplitude detecting unit 126 detects amplitudes of the simulated image signals output from the complex filter circuit 125. Then, based on the detected amplitudes, a filter control unit 127 sets element values in a register group within an element value control unit 1252 so as to decrease the amplitudes of the IF band simulated image signals. The element value control unit 1252 adjusts and controls the element values of an I filter 1251I and a Q filter 1251Q within the complex filter circuit 125, thereby trying to compensate for an IQ mismatch in the complex filter circuit 125.
A second related art for improving the image rejection ratio is disclosed in Patent Literature 2. In the second related art disclosed in Patent Literature 2, as shown in FIG. 13, a calibration signal of an RF band is generated from a calibration signal source 131 under the control of a calibration control circuit 130. The calibration signal is input to a quadrature mixer 133 through a calibration signal switch 132. Further, an I component signal and a Q component signal, which are obtained by frequency conversion into an IF band, are output. Then, the I component signal is input to a filter mismatch calibration circuit 138 through an I component signal path 134 and an ADC 136, and the Q component signal is input to a filter mismatch detecting circuit 139 through a Q component signal path 135 and an ADC 137.
The filter mismatch detecting circuit 139 detects a mismatch occurring in the quadrature mixer 133, the I component signal path 134, the Q component signal path 135, and the ADCs 136 and 137.
Based on the detected mismatch, a tap coefficient calculating circuit 140 calculates a tap coefficient of the filter mismatch calibration circuit 138, and updates the tap coefficient of the filter mismatch calibration circuit 138 with the calculated tap coefficient.
Thus, the image rejection ratio between the I component signal and the Q component signal is improved.
Further, Patent Literature 3 discloses, as a third related art, a radio receiving circuit having the following configuration. That is, a pair of intermediate frequency signals are generated by converting the frequency of an input signal using a mixer for every two local oscillation signals having the same frequency and shifted in phase by 90 degrees from each other, and the intermediate frequency signal corresponding to the local oscillation signal with a phase delay of 90 degrees is shifted by a predetermined amount of phase. After that, the phase-shifted intermediate frequency signal and the other intermediate frequency signal are added to be output as an intermediate frequency signal. The radio receiving circuit disclosed as the third related art is configured to cancel the image signal component by using a reference image signal of an image frequency, which is a frequency corresponding to a difference between a local oscillation frequency and an intermediate frequency, as an input signal supplied to the mixer, in an adjustment mode of the radio receiving circuit for cancelling the image signal component generated due to variations in the paths through which the two intermediate frequency signals pass.